“Even though I knew Elliot was creepy,” Dtugg – who agreed to speak on the condition that he be identified solely by his username – told the Guardian, “it was one of the most shocking things that has ever happened to me.”

Dtugg had tangled with Rodger several times on the boards of bodybuilding.com’s “misc” section, and had attempted to give advice to a man he saw as clearly troubled. Others on the forum didn’t know what to make of Rodger.

It was a space given to flamewars and trolling, but they saw Rodger posting things like “Men shouldn't have to look and act like big, animalistic beasts to get women. The fact that women still prioritize brute strength just shows that their minds haven't fully evolved”; “Women are not drawn to indicators of evolutionary fitness. If they were, they'd be all over me”; and “Never insult the style of Elliot Rodger. I’m the most stylish person in the world. Just look at my profile pic. That’s just one of my fabulous outfits. The sweater I’m wearing in the picture is $500 from Neiman Marcus.”

They assumed he was putting on an act. But to Dtugg, something felt wrong.

“I can easily see someone who is really like that making such a video,” he posted on 19 May, just four days before the stabbings and shootings in Isla Vista, in a thread he started about Rodger’s YouTube channel.

The forum Dtugg frequented was part of a wider community of semi-social circles overlapping on websites and message boards – online cliques with their own vernaculars and labyrinthine hierarchies. For a while before the shooting it was Rodger’s world too.

Many of these circles revolved around the social-news site Reddit. There are the PUAs – pick-up artists – whose existence was chronicled in Neil Strauss’s 2005 book The Game, and whose numbers have grown exponentially since. They hang out on the Seduction subreddit (172,473 subscribers) and places like pick-up-artist-forum.com (148,511 members), where they exchange tips on how to build confidence and how to get women into bed.

Then there are the “Red Pill” people, who cluster around a large subreddit forum of the same name and blend pick-up artistry with “men’s rights” advocacy. The name refers to the cult film The Matrix, in which the main character is asked to choose between taking a red pill and waking up to the truth and taking a blue pill that will leave him ignorant. Its constitution states that “women are irrational and inconsistent” and “machiavellian in nature”. It has 53,538 subscribers.

Another subreddit, “ForeverAlone”, is where “incels” hang out. Incel is short for “involuntary celibates”: people who define themselves solely by their inability to sleep with women. It is a definition Elliot Rodger used for himself. Forever Alone has 33,278 subscribers.

More serious Incels congregate on the innocuously named love-shy.com, where forums feature posts like “It upsets me, seeing all the Hot Babes I can't have sex with”. Love-shy has 3,689 members.

Beneath some of these forums and subreddits was PUAhate.com, which one user described as “one of the few truly ‘Red Pill’ communities”. Founded to satirise and discredit pick-up artists, it became a place where sexually frustrated men could go to vent and share pseudo-scientific theories about women. In the spring of 2013, Elliot Rodger found it.

On PUAhate, he wrote in his sprawling manifesto, he had discovered “a forum full of men who are starved of sex, just like me”. What he read, he continued, “confirmed many of the theories I had about how wicked and degenerate women really are”.

“The moderation policy was very laissez-faire,” one PUAhate.com user, who asked to be referred to as Tom, told the Guardian. “There was racism; definitely a lot of misogyny. Elliot Rodger’s type of comment wouldn’t have been uncommon.”

On all these sites, too, was an incendiary mix of people who were deadly serious and those who were trolling – egging the serious people on, for kicks. “It was a mix of people who were really struggling and had never had a kiss, to guys who who were there to get a rise,” said Tom.

Rodger’s manifesto, and the videos he posted to YouTube, were liberally scattered with the lingo of PUA and the Red Pill people. “There is something mentally wrong with the way [women’s] brains are wired,” he wrote, echoing the Red Pill constitution. “They are incapable of reason or thinking rationally.”

Later he echoed classic PUA lingo, describing himself trying to act “cocky and arrogant” – a phrase he repeated a few lines later. To put people down, he described them as “betas” – referring to “beta males”, another PUA trope.

The community even had a phrase for what Rodger did: “Going Sodini”, for George Sodini, who in 2009 killed three women and injured nine other people before killing himself, and who had written extensively online about being rejected by women.

“Many people commented that it was inevitable something like this would happen [again],” Tom said. “Maybe people thought it was humour, maybe no one thought someone would actually do it. But you have a site that cultivates these type of thoughts, and men who have this type of rage.”

Another user, who goes by the name Hypnoreality, was more succinct. “The site was a ticking time bomb,” he said.

Harris O’Malley is a former member of the PUA scene. He now runs a blog called Dr Nerdlove, where he tries to provide a counterpoint to what he calls the “toxic masculinity” of such communities. “[Rodger] found a lot of fellow travellers on Red Pill, and especially on PUAhate,” he told the Guardian.

“There's an amplification effect,” he said. “It spurs people on, and people who come in with disagreements tend to be chased out. It's made clear that dissenting opinions aren't welcome, especially ones that go against the dominant narrative.”

After news of the shooting broke but before PUAhate was taken offline, some users posted adulatory comments to Rodger’s public profile. A couple called him a hero.

But one user of PUAhate who had more than a thousand posts on the site contacted me via reddit to complain that the site was “being depicted as a place where bitter men sat around discussing their hatred of women”. He told me that discussion on the site was usually “more light-hearted than violent”.

He didn’t wish to give his real name. On Reddit, he goes by the username “ElliotRodgerIsAGod”.